Monofluoro-polyphosphates



United States Patent 3,420,620 MONOFLUORO-POLYPHOSPHATES Robert E. Mesmer, Oak Ridge, Tenn., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Aug. 11, 1966, Ser. No. 571,705 US. Cl. 2350 18 Claims Int. Cl. C01b 25/30; C01c 1/28 This invention relates to monofluoro-polyphosphates and the methods of preparing the same. More particularly, it relates to alkali metal and ammonium monofluoro-tripolyphosphate and the production of the same from a trimetaphosphate and a fluoride ion source.

The new alkali metal and ammonium monofluorotripolyphosphates of this invention have the heavy metal sequestering properties of the tripolyphosphate and provide a relatively nontoxic fluorine source. Such properties make them useful in the production and formulation of dentifrices. Additionally the alkali metal and ammonium monofluoro-tripolyphosphates can readily be hydrolyzed to the corresponding monofluoropyrophosphates and monofluoro-orthophosphates both of which are useful in dentifrice applications.

In accordance with this invention, it has been found that the presence of a fluoride ion under controlled conditions causes the cleavage of trimetaphosphate rings to form the new compounds, the alkali metal monofluorotripolyphosphates. It is believed this is surprising because other halide ions do not react with the trimetaphosphate ring compounds in this manner under similar reaction conditions.

In particular, it has been found that alkali metal and ammonium trimetaphosphates react in aqueous mediums with fluoride ions at reaction temperatures of about 20 C. to about 100 C. and at pI-Is of from about 2 to about 11 to produce monofluoro-tripolyphosphates of the following formula M XP O F (Formula 1) wherein M is selected from the group consisting of ammonium and alkali metal and X is selected from the group consisting of hydrogen, ammonium, and alkali metals.

Although any of the alkali metals and ammonium can be used, it is generally preferred to use sodium, potassium, or ammonium in the practice of this invention, with sodium being especially preferred. In subsequent paragraphs, for the sake of brevity, the invention for the most part will be described with particular reference to the use of alkali metal ions and alkali metal compounds to produce alkali metal monofluoro-tripolyphosphates; but, it will be understood that in each instance where men tioned part or all of the alkali metal ions or compounds can be replaced with ammonium to produce the corresponding ammonium monofluoro-tripolyphosphate or mixed ammonium alkali metal monofluoro-tripolyphosphates.

When suflicient alkali metal ions are present from an alkali metal source such as an alkali metal base or an alkali metal salt which ionizes in the reaction medium to yield an alkali metal ion, then a tetra-alkali metal monofluoro-tripolyphosphate salt is produced which is represented by the following equation M P O F (Formula 2) wherein M is as defined in Formula 1.

One of the preferred methods of producing the tetraalkali metal monofluoro-tripolyphosphates is to use an alkali metal fluoride as the source for providing the fluoride ions and alkali metal ions in addition to those present in the reaction medium from the alkali metal tripolyphosphate. 1

The alkali metal monofluoro-tripolyphosphates have been found to undergo hydrolysis to form a mixture of alkali metal monofluoroorthophosphates, alkali metal monofiuoro-pyrophosphates, alkali metal orthophosphates, and alkali metal pyrophosphates. If the hydrolysis is allowed to continue to completion, each mole of alkali metal monofluoro-pyrophosphate hydrolyzes to form one mole of alkali metal monofluoro-orthophosphate and one mole of alkali metal orthophosphate. Similarly, each mole of alkali metal pyrophosphate hydrolyzes to form two moles of orthophosp'hates. Therefore, if it is desired the alkali metal monofluoro-tripolyphosphates produced by this invention can be used to prepare a mixture of alkali metal monofluoro-orthophosphate and alkali metal orthophosphate having a 1:3 molar ratio, respectively. Since the alkali metal orthophosphate is a raw material for the manufacture of alkali metal trimetaphosphate, it can be selectively crystallized from the reaction medium and recycled, if desired, to produce an alkali metal trimetaphosphate while the alkali metal monofluoro-orthophosphate can be selectively crystallized and used in dentifrice formulations. The alkali metal monofluoro-tripolyphosphates can be recovered from the solution in crystalline form by known recovery techniques such as selective crystallization, and the like; however, in most instances, the solutions will be used without separating the components because all components contained therein are useful in dentifrices.

It is necessary to have an aqueous reaction medium to prepare alkali metal monofluoro-tripolyphosphate. It has been found that when molten alkali metal trimetaphosphate is reacted with molten sodium fluoride the product obtained is not alkali metal monofluoro-tripolyphosphate but is a mixture of alkali metal monofluoro-orthophosphate and alkali metal monofluoro-pyrophosphate. Any material which will ionize in the reaction medium to form a fluoride ion can be used in the practice of this invention. It is generally preferred, however, to use a source selected from the group consisting of hydrogen fluoride, alkali metal fluorides, and mixtures thereof. Specific examples of suitable fluorides include sodium fluoride, lithium fluoride, potassium fluoride and ammonium fluoride.

The pH of the aqueous reaction medium is maintained with the range of from about 2 to about 11 in the practice of this invention. At a pH of below about 2, the hydrolysis of the alkali metal monofluoro-tripolyphosphate to the before-mentioned mixture of salts occurs almost immediately after the formation of the alkali metal monofluorotripolyphosphate. It is, therefore, preferred, particularly when larger amounts of alkali metal monofluoro-tripolyphosphates are desired, to hold the pH of the reaction medium at about 4 or above.

It is also necessary to prevent the pH from exceeding about 11 and preferably about 10 in the practice of this invention because to obtain a pH greater than about 11, a strong base must be used and the alkali metal trimetaphosphate ring will be cleaved and an alkali metal tripolyphosphate will be formed instead of the preferred alkali metal monofluoro-tripolyphosphate. It is preferred, therefore, to use a pH from about 4 to about 10. In most instances, the pH of the reaction medium will be within the desired range when either hydrogen fluoride or the alkali metal fluorides are used as the sou-roe of the fluoride ion. If hydrogen fluoride is used, however, and a tetra-alkali metal monofluoro-tripolyphosphate is desired, it is necessary to use an additional source of an alkali metal ion.

As heretofore mentioned, this invention is conducted at temperatures from about 20 C. to about 100 C. Temperatures below about 20 C. are not used because the reaction between the alkali metal trimetaphosphate and the fluorine source is too slow to form any appreciable amount of alkali metal monofluoro-tripolyhposphate within a reasonable time. At temperatures above 100 C. hydrolysis of the alkali metal trimetaphosphate is rapid and yields undesired products (other alkali metal phosphates), which adversely affect the production of alkali metal monofluoro-tripolyphosphates. Additionally, any alkali metal monofluoro-tripolyphosphate which is formed is hydrolyzed almost immediately into the mixture of the various salts as heretofore described. It is, therefore, preferred when practicing this invention to hold the temperature of the reaction medium from about 60 C. to about 90 C. It is still further preferred to hold the temperature between about 75 C. and 85 C. to maximize the alkali metal monofluoro-tripolyphosphate production.

Any concentration of reactants and/or of products in the reaction medium can be used and still achieve the benefits of this invention. The concentration of reactants, therefore, is not critical; however, it is preferred to use an aqueous slurry of alkali metal tripolyphosphate having an alkali metal trimetaphosphate content of below about 25% by weight and more preferably having an alkali metal trimetaphosphate content of from about 5% to about 20% by weight. Although various concentrations of the fluorine source can be used in the practice of this invention, it is also generally preferred to utilize aqueous mediums, that is, aqueous solutions or slurries having a concentration of from about 0.1 to about 6.0 moles of fluorine source per liter of aqueous medium with those having a concentration of from about 0.5 to about 3.0 moles of fluorine source per liter of aqueous medium being especially preferred. By using the concentrations as herein specified conventional equipment can be used to obtain a uniform distribution of reactants throughout the reaction mass. Therefore, the problems that are associated with some chemical process such as overheating, nonuniformity of products, and the like are largely avoided by the use of the preferred concentrations.

The practice of this invention is also independent of the order of addition. The fluorine source can be added to an aqueous solution or slurry of alkali metal trimetaphosphate having a temperature of from about 20 C. to about 100 C. with satisfactory results. The pH of such an aqueous solution or slurry will normally be within the pH range of 2-11. Similarly, the alkali metal trimetaphosphate can be added to an aqueous solution or slurry of the fluorine source with equally good results. It is to be noted, however, that if an aqueous solution of hydrogen fluoride is used in the practice of this invention it has a low pH; therefore, it is necessary to adjust the pH within the range as specified herein prior to the addition of the alkali metal trimetaphosphate in order to avoid excessive hydrolysis of the alkali metal monofluoro-tripolyphosphate.

Any molar ratio of the before-mentioned reactants can be used in the practice of this invention and still achieve some of the benefits of this invention. For example, molar ratios of fluorine to alkali metal trimetaphosphate as low as 1:10 or even lower such as 1:20 and 1:100 can be used; however, it is preferred that the molar ratio of fluoride ion to trimetaphosphate be greater than about 1.0 to increase the yield of alkali metal monofluoro-tripolyphosphate. It is especially preferred to use molar ratios of fluoride to the trimetaphosphate of from about 1.0 to about 4.0. Although molar ratios above about 4.0 can be used, such as :1 or :1, these needlessly add to the cost of the process.

Illustrative of the new compounds which can be produced by the process of this invention are compounds represented by the following formulas:

To specifically illustrate the process of this invention, the following examples are presented. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 About 1,500 parts of a 20% aqueous solution of sodium trimetaphosphate and about 800 parts of a 10% aqueous solution of sodium fluoride are charged to a conventional reactor equipped with a heating and cooling means. The pH of the aqueous medium is adjusted to about 7.5 with the addition of a 25% aqueous solution of sodium hydroxide and is heated to about C. within about 1 minute.

About 15 minutes after the temperature reaches about 80 C. a sample of the aqueous solution analyzed for phosphorous content and by P nuclear magnetic resonance (nmr) shows that about 70% phosphorous is present as monofluoro-tripolyphosphate and about 30% is present as monofluoro-tripolyphosphate. Within 3 to 4 hours essentially all of the trimetaphosphate is converted to monofluoro-tripolyphosphate. Under these reaction conditions, however, some of the monofluoro-tripolyphosphate hydrolyzes to the lower phosphates within the time period required to convert all of the trimetaphosphate to monofiuoro-tripolyphosphate.

Maintaining the reaction conditions for about 9 to 10 additional hours results in nearly complete hydrolysis of the alkali metal monofluoro-tripolyphosphate and nmr analysis indicates about /3 of the phosphorous appears as monofluoro-orthophosphate and none as trimetaphosphate. In view of the above results, it can be seen that in the process of this example (1) all of the original alkali metal trimetaphosphate reacts with the fluoride ion to form monofluoro-tripolyphosphate and (2) none of the P-F bonds originally formed in the ring cleavage subsequently hydrolyze and (3) none of the phosphates other than the trimetaphosphates will rcact directly with the fluoride ion.

EXAMPLE 2 Using essentially the same procedure as in Example 1 only holding the reaction medium at 25 C., the reaction rate is decreased so that about 8 days are required to obtain about 70% conversion of the trisodium trimetaphosphate to tetrasodium monofluoro-tripolyphosphate; however, some of the monofluoro-tripolyphosphate hydrolyzes during this period.

EXAMPLE 3 About 2,000 parts of an aqueous slurry containing about 15% potassium trimetaphosphate and about 600 parts of an aqueous solution containing about 10% hydrogen fluoride are charged into a conventional stirred reactor. The pH is adjusted to about 4 with the addition of a 20% aqueous solution of potassium hydroxide. The temperature of the solution is raised to about 75 C. and after about 1 hour about 80% of the potassium trimetaphosphate is converted to potassium monofluoro-tripolyphosphate as shown by standard elemental analysis techniques and by P nuclear magnetic resonance.

What is claimed is:

1. A process for producing a polyphosphate of the formula, M XP O F, wherein M is selected from the group consisting of ammonium and alkali metals and wherein X is selected from the group consisting of hydrogen, ammonium and alkali metals comprising reacting a substance yielding fluoride ions and a trimetaphosphate selected from the group consisting of alkali metal trimetaphosphates and ammonium trimetaphosphate, in an aqueous solution and at a temperature of from about 20 C. to about 100 C. and at a pH of from about 2 to about 11.

2. A process according to claim 1 wherein said fluoride ions are obtained from a fluoride ion source selected from the group consisting of hydrogen fluoride, alkali metal fluoride, ammonium fluoride, and mixtures thereof.

3. A process according to claim 2 wherein said pH temperature and pH are maintained until a major portion of said polyphosphate is hydrolyzed to alkali metal monofluoro-orthophosphate and alkali metal orthophosphate.

4. A process according to claim 2 wherein said temperature is from about 50 C. to about 90 C.

5. A process according to claim 4 wherein said trimetaphosphate is selected from the group consisting of trisodium trimetaphosphate, tripotassium trimetaphosphate, and triam-monium trimetaphosphate.

6. A process according to claim 5 wherein said alkali metal trimetaphosphate is tripotassium trimetaphosphate.

7. A process according to claim 5 wherein said fluoride ion source is potassium fluoride.

8. A process according to claim 5 wherein said alkali metal trimetaphosphate is in the form of an aqueous slurry containing from about 5% to about 20% by weight of trisodium trimetaphosphate and said fluoride ion source is in an aqueous medium wherein said medium has a concentration of from about 0.1 to about 6.0 moles or" fluoride ion source per liter of aqueous medium and wherein the molar ratio of said fluoride ion source to said trisodium trimetaphosphate is from about 1:1 to about 4:1. r

9. A process according to claim 8 wherein said fluoride ion source is hydrogen fluoride.

10. A process according to claim 8 wherein said fluoride ion source is sodium fluoride and wherein said polyphosphate is tetrasodium monofluoro-tripolyphosphate.

11. A compound of the formula, M XP O' F, wherein M is selected from the group consisting of ammonium and alkali metals and wherein X is selected from the group consisting of hydrogen, ammonium and alkali metals.

12. A compound according to claim 11 wherein'M is selected from the group consisting of sodium, potassium, and ammonium and X is selected from the group consisting of hydrogen, ammonium, sodium, and potassium.

13. A compound according to claim 12 wherein M is ammonium and X is hydrogen.

14. A compound according to claim 12 wherein M is sodium and X is hydrogen.

15. A compound according to claim 12 wherein M is potassium and X is hydrogen.

16. A compound according to claim 12 wherein M and X are sodium.

17. A compound according to claim 12 wherein M and X are potassium.

18. A compound according to claim 12 wherein M and X are ammonium.

References Cited UNITED STATES PATENTS 2,481,807 9/ 1949 Anderson 23-50 3,372,984 3/1968 Metcalf et al. 23-107 FOREIGN PATENTS 1,344,497 10/1963 France.

752,819 8/1951 Germany.

EARL C. THOMAS, Primary Examiner. HERBERT T. CARTER, Assistant Examiner.

US. Cl. X.R. 23-107 

1. A PROCESS FOR PRODUCING A POLYPHOSPHATE OF THE FORMULA, M3XP3O9F, WHEREIN M IS SELECTED FROM THE GROUP CONSISTING OF AMMONIUM AND ALKALI METALS AND WHEREIN X IS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN, AMMONIUM AND ALKALI METALS COMPRISING REACTING A SUBSTANCE YIELDING FLUORIDE IONS AND A TRIMETAPHOSPHATE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL TRIMETAPHOSPHATES AND AMMONIUM TRIMETAPHOSPHATE, IN AN AQUEOUS SOLUTION AND AT A TEMPERATURE OF FROM ABOUT 20*C. TO ABOUT 100*C. AND AT A PH OF FROM ABOUT 2 TO ABOUT
 11. 11. A COMPOUND OF THE FORMULA, M3XP3O9F, WHEREIN M IS SELECTED FROM THE GROUP CONSISTING OF AMMONIUM AND ALKALI METAL AND WHEREIN X IS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN, AMMONIUM AND ALKALI METALS. 